Convert follow path animation to keyframes
Well, the answer provided in the comments is the answer to your question though I am not sure this is the best way to adjust the timing of a follow path animation.

The path animation is animated using the 'evaluation time' value when the corresponding curve is selected. Make sure to delete the modifier which is associated with the curve by default in order to set custom keyframes. If you are just planning to offset or adjust the speed of the animation you can also play with the values of the modifier

Below you can see where to find the value and how to delete the modifier.



This way you can offset the animation in a non-destructive way.